A long turbine shaft that interconnects several turbines, such as steam operated turbines, may suffer from torsional oscillations at one or more resonance frequencies of the shaft. Torsional resonance oscillations may lead to a breakdown of the turbine shaft and it is therefore necessary to avoid such oscillations and be able to detect them when they occur so that the torsional resonance oscillations or the turbines can be stopped. Torsional oscillations of the shaft at the rotor of the generator affects the electric power that is output from the generator and appear as variations in the current and voltage. Also, an electric power transmission system connected to the generator output terminals may induce electric energy to the shaft through the generator and thereby contribute to torsional oscillations in the turbine-generator shaft. Especially in electric transmission systems having reactive power compensation, the electrical transmission system may show resonance frequencies matching the resonance frequencies of the mechanical system so that the electric transmission system starts to interact with the torsional resonance oscillations of the shaft.
Different suggestions for counteracting torsional resonance oscillations can be found in the prior art, for example systems that monitors the generator output current to detect the torsional resonance oscillations.
U.S. Pat. No. 4,733,340 (Mase et al) describes a system for stabilizing torsional oscillations in a turbine-generator shaft. In this system, the output current of the generator is measured at the output terminals of the generator. From the torsional resonance frequency (fm in Mase et al) and the generator frequency (f1 in Mase et al), a sub-synchronous and a super-synchronous resonance frequency of a current signal can be determined, which sub-synchronous and a super-synchronous resonance frequencies correspond to the resonance frequency of the torsional resonance oscillation. The sub-synchronous frequency (f1−fm in Mase et al) component of the current and the super-synchronous frequency (f1+fm in Mase et al) component of the current signal are filtered out by means of a respective, first and second, bandpass filter. The respective signal at the sub-synchronous frequency and the super-synchronous frequency is detected.
A drawback with using filters is that it is difficult to tune the filters correctly. A too wide bandpass filter will be sensitive to disturbances, whereas a narrow bandpass filter may not give accurate measurements of the magnitude if the resonance frequency is not known exactly, or if the filter cannot be tuned exactly.
Another drawback with such monitoring of the generator terminal currents are that the generator terminal currents varies with the impedance of the electric transmission system. Thus, variations within the electric transmission system may affect the current measurements, and the amplitude of the current frequencies may vary as a result of variations of the impedance of the electric transmission system.
U.S. Pat. No. 4,331,882 describes a system for detecting sub-synchronous and super-synchronous electrical signals in an electric power system. The electrical signals being used are current or voltage signals (col. 2, line 25-30 in U.S. Pat. No. 4,331,882). The obtained electrical signals are rectified and filtered by means of tuned filters. The filters are tuned to filter out the sub-synchronous and super-synchronous resonance components of the electrical signals. The filter detector provides an output that can be used to actuate an alarm and as input to a control circuit.
As mentioned in relation to U.S. Pat. No. 4,733,340 (Mase et al), a drawback with filtering out frequency components by means of a bandpass filters (as in U.S. Pat. No. 4,331,882) is that the filters need to cover a sub-band having a certain size since the torsional resonance oscillations may only be approximately known. There is a risk that other disturbances enter into such a sub-band and therefore will be detected as torsional resonance oscillations if the filters are not exactly tuned, while in fact these detected frequency disturbances will not cause harm to the shaft. Thus, there is a risk that the turbine-generator system is stopped when it is not necessary.
Moreover, the frequency of the generator terminal output current and voltage may vary slightly during operation since the frequency of the electric transmission network may vary. Thus, the sub-synchronous filter, for example, has to cover a certain extra bandwidth of the sub-synchronous frequency range and may therefore also register non-resonance signals as resonance signals.
The scientific paper “Online Estimation of Subsynchronous Voltage Components in Power Systems” (Bongiorno et al), IEEE Transactions on Power Delivery, vol. 23, no. 1, January 2008, describes methods for monitoring sub-synchronous components in the generator terminal voltage. The document discuss digital sampling of the generator voltage and detecting frequency variations that correspond to the torsional oscillations. The paper discuss detecting methods using digital FFT (Fast Fourier Transform) or Kalman filtering. Although such digital analysis may be more reliable than analogue bandpass filtering, there is a risk that a detected sub-synchronous frequency is not related to a torsional resonance oscillation, since the torsional resonance frequencies are only approximately known. Moreover, other disturbances of the frequency content may exist, for example as created by high power machines, such as for example an electric arc furnace in a steel production facility, which may create disturbances close to the torsional oscillation resonance frequencies.
Erroneously registering a signal as indicative of resonance may lead to a shutdown of the turbine-generator system when there is no need doing so. Shutting down a power production system is costly and should be avoided if it is not necessary.
Thus, there is a need to monitor torsional oscillation of a turbine-generator system with a high accuracy and reliability, so as not to unnecessarily shut down a power producing turbine-generator system.